And broke the link to the corrupt toast data:UPDATE events SET data = '' WHERE id = 9035;

This cleared the corruption and allowed me to upgrade GitLab.

What I’ve not managed to do yet is install and run pg_repack to clean up the toast tables. I know nothing about Postgres but this is apparently required otherwise the Postgres VACUUM will fail. I found the following link about pg_repack and GitLab but that is as far as I’ve got.